7 A device class describes a type of device, like an audio or network
8 device. The following device classes have been identified:
10 <Insert List of Device Classes Here>
13 Each device class defines a set of semantics and a programming interface
14 that devices of that class adhere to. Device drivers are the
15 implementation of that programming interface for a particular device on
18 Device classes are agnostic with respect to what bus a device resides
24 The device class structure looks like:
27 typedef int (*devclass_add)(struct device *);
28 typedef void (*devclass_remove)(struct device *);
30 See the kerneldoc for the struct class.
32 A typical device class definition would look like:
34 struct device_class input_devclass = {
36 .add_device = input_add_device,
37 .remove_device = input_remove_device,
40 Each device class structure should be exported in a header file so it
41 can be used by drivers, extensions and interfaces.
43 Device classes are registered and unregistered with the core using:
45 int devclass_register(struct device_class * cls);
46 void devclass_unregister(struct device_class * cls);
51 As devices are bound to drivers, they are added to the device class
52 that the driver belongs to. Before the driver model core, this would
53 typically happen during the driver's probe() callback, once the device
54 has been initialized. It now happens after the probe() callback
55 finishes from the core.
57 The device is enumerated in the class. Each time a device is added to
58 the class, the class's devnum field is incremented and assigned to the
59 device. The field is never decremented, so if the device is removed
60 from the class and re-added, it will receive a different enumerated
63 The class is allowed to create a class-specific structure for the
64 device and store it in the device's class_data pointer.
66 There is no list of devices in the device class. Each driver has a
67 list of devices that it supports. The device class has a list of
68 drivers of that particular class. To access all of the devices in the
69 class, iterate over the device lists of each driver in the class.
74 Device drivers are added to device classes when they are registered
75 with the core. A driver specifies the class it belongs to by setting
76 the struct device_driver::devclass field.
79 sysfs directory structure
80 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~
81 There is a top-level sysfs directory named 'class'.
83 Each class gets a directory in the class directory, along with two
84 default subdirectories:
92 Drivers registered with the class get a symlink in the drivers/ directory
93 that points to the driver's directory (under its bus directory):
99 `-- usb:usb_mouse -> ../../../bus/drivers/usb_mouse/
102 Each device gets a symlink in the devices/ directory that points to the
103 device's directory in the physical hierarchy:
108 | `-- 1 -> ../../../root/pci0/00:1f.0/usb_bus/00:1f.2-1:0/
114 struct devclass_attribute {
115 struct attribute attr;
116 ssize_t (*show)(struct device_class *, char * buf, size_t count, loff_t off);
117 ssize_t (*store)(struct device_class *, const char * buf, size_t count, loff_t off);
120 Class drivers can export attributes using the DEVCLASS_ATTR macro that works
121 similarly to the DEVICE_ATTR macro for devices. For example, a definition
124 static DEVCLASS_ATTR(debug,0644,show_debug,store_debug);
126 is equivalent to declaring:
128 static devclass_attribute devclass_attr_debug;
130 The bus driver can add and remove the attribute from the class's
131 sysfs directory using:
133 int devclass_create_file(struct device_class *, struct devclass_attribute *);
134 void devclass_remove_file(struct device_class *, struct devclass_attribute *);
136 In the example above, the file will be named 'debug' in placed in the
137 class's directory in sysfs.
142 There may exist multiple mechanisms for accessing the same device of a
143 particular class type. Device interfaces describe these mechanisms.
145 When a device is added to a device class, the core attempts to add it
146 to every interface that is registered with the device class.